Abrasion and antifog-resistant optical element

ABSTRACT

An optical element is provided having a durable abrasion-resistant, antifog coating applied to at least one surface thereof by coating an optical element comprised of an inorganic or organic transparent base with a transparent abrasion-resistant antifog coating consisting of a lightly cross-linked polyvinyl alcohol, the desired degree of cross-linking being obtained by utilizing a combination of zirconium nitrate and formaldehyde as cross-linking agents.

This is a continuation of application Ser. No. 578,793 filed May 19,1975 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The invention is in the field of optical elements based upon eitherinorganic and organic base elements. Such optical elements as windows,eyeglasses, goggles, faceshields, binoculars and optical instruments areknown to be subject to fogging, that is the condensation of moisture onsuch optical surfaces when these surfaces are cooled below thetemperature of the surrounding air. Since the heat transfer orconduction of inorganic glass optical elements is greater than that ofan organic glass optical element, the tendency for fogging is generallygreater on an inorganic glass optical element. However, under conditionsof extreme humidity, even organic glass lenses are subject to fogging ormoisture condensation on their surfaces.

2. Description of the Prior Art:

A review of prior art attempts to provide an antifogging substrate isprovided in U.S. Pat. No. 3,700,487 wherein an antifogging coating isdisclosed for such optical elements as glass, polycarbonate,polymethylmethacrylate, diethylene glycol bis allyl carbonate andcopolymers thereof, methylmethacrylate and metal. The teaching of thispatent includes a process for the preparation of the surface of theoptical element substrate for subsequent coating so that adequatebonding of the antifogging coating can be obtained. Preparation of thesurface is achieved by dipping the optical element into a solution of analkyl titanate or by a hydrolytic surface treatment in which analcoholic or aqueous caustic such as sodium or potassium hydroxide isapplied to the optical element prior to coating with the antifog coatingdisclosed. Other methods of obtaining adequate adhesion of the antifogcoating are disclosed as the application of a bonding coat of polyvinylbutyral or the application of a nylon lacquer sold under the trademark"Milvex 4000" manufactured by General Mills. The antifog coating whichis subsequently applied, for instance, to a polycarbonate substrate,consists of polyvinyl alcohol in solution in combination with hydriodicacid as a cross-linking catalyst. The solution pH is approximately 2.3.

In. U.S. Pat. No. 3,484,157, there is disclosed an abrasion-resistantoptical element having a directly-adhering, transparent surface coatingcomprising a vinyl polymer cross-linked with a dialdehyde cross-linkingagent. Such dialdehydes as glyoxal, pyruvic aldehyde, 2hydroxyadipaldehyde or glutaraldehyde can be used.

SUMMARY OF THE INVENTION

A durable abrasion or scratch-resistant, antifog coating is provided foran optical element of an inorganic or organic optical element by theapplication of a polyvinyl alcohol coating lightly cross-linked with acombination of zirconium nitrate and formaldehyde as cross-linkingagents. In the process of the invention, adequate bonding of the antifogcoating of the invention to various substrates can be obtained, forinstance, by an hydrolytic surface treatment of the optical elementsurface to be coated in which an aqueous or alcoholic caustic such assodium or potassium hydroxide is applied thereto. The process of theinvention provides unexpectedly improved control of the degree of cureof the polyvinyl alcohol antifog coating wherein a satisfactory balancebetween the water resistance of the coating, as determined by a test inwhich the coating is abraded under water, and the antifog resistance ofthe coating, as determined by a test in which the coated optical elementis exposed to high humidity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, there is provided a lightlycross-linked polyvinyl alcohol coating on the surface of a transparentbase optical element, said coating being applied to said optical elementonly after the surface of the optical element has been initially treatedto provide a permanent bonding layer therebetween. The durable,abrasion-resistant antifog coating of the invention can be applied to anoptical element substrate of glass, plastic or metal and the coatingprovides a permanent antifogging surface layer which isabrasion-resistant as well. The antifog coating can be applied to one ormore surfaces of an optical element, for instance, to one or moresurfaces of a lens. The base optical element can be a polymericsubstrate formed of polycarbonates exemplified by the product sold byGeneral Electric Company as "Lexan" which is a poly-(4,4'-dioxydiphenyl-2,2-propane carbonate) of the formula: ##STR1## n =about 50 to about 200. The polymer is prepared by condensing bisphenol-A(4,4'-dihydroxy diphenyl-2,2-propane) with phosgene, in the presence ofa hydrogen chloride acceptor such as an amine base. Other embodiments ofthe base optical element substrate are, but are not limited to, allyldiglycol carbonate methyl methacrylate, styrene copolymer, celluloseesters such as cellulose acetate and cellulose acetate butyrate.

In general, the practice of the invention provides for initiallypreparing the surface of a substrate to insure adequate bonding of theantifog coating, for instance, by treating the surface with an aqueousor alcoholic caustic such as sodium or potassium hydroxide so as tocause a degree of hydrolysis in the surface of the base optical element.Other adhesion promoting treatments can also be used.

The hydrolysis treatment can be applied to the base optical element bydipping the base optical element into a solution of an aqueous oralcoholic caustic containing an alkali metal hydroxide such as sodium orpotassium hydroxide present at a concentration of about 10 percent toabout 20 percent. The treatment can be conducted at a temperature ofabout 20° C to about 70° C and the time in which the base opticalelement is exposed to said caustic can be about 1 hour to about 24hours. The base optical element subsequent to treatment in said causticis rinsed in water and dried at a temperature of about 20° C to about50° C.

The polyvinyl alcohol which is applied to the base optical element ofthe invention is prepared as a solution in water or water and an alcoholhaving 2 to 6 carbon atoms present in a proportion of about 20 percentto about 60 percent. The concentration of the polyvinyl alcohol solutioncan be about 3 percent to about 5 percent. It is desirable to include asurface active agent or wetting agent in the solution of polyvinylalcohol to provide more complete solution of the polyvinyl alcohol andoptimum wetting of the polyvinyl alcohol solution when this is appliedto the base optical element to be coated. The surface active agent canbe a non-ionic, cationic or anionic type material such as are well-knownto those skilled in the art.

The polyvinyl alcohol can be either the so-called partially hydrolyzedor fully hydrolyzed commercially available grade, said polyvinyl alcoholbeing derived from polyvinyl acetate by hydrolysis so as to produce thedesired proportion of polyvinyl alcohol. Polyvinyl alcohols are producedcommercially in grades varying between 65 and 98 percent degree ofhydrolysis of the polyvinyl acetate starting material. The fullyhydrolyzed grade is preferred for use in the practice of the invention.Polyvinyl alcohol is also available commercially in various molecularweights varying between about 14,000 molecular weight (weight average)to about 115,000 molecular weight (weight average).

It has been found that the commercially available grades of polyvinylalcohol having a weight average molecular weight of at least about85,000 and higher can be utilized in the practice of the invention. Withthe low molecular weight grades, a higher concentration of polyvinylalcohol in the coating solution can be obtained as compared to thehigher molecular weight grades of polyvinyl alcohol. The molecularweight of the polyvinyl alcohol is important in the process of theinvention primarily as a means of controlling the viscosity of thepolyvinyl alcohol solution and thereby the coating thickness of thedurable, abrasion-resistant antifog coating on the optical element ofthe invention.

In the process of the present invention, the base optical element iscoated with a dried polyvinyl alcohol coating at a thickness of about0.5 to about 3 microns. Although thinner or thicker coatings can beapplied which will provide durability as well as resistance to abrasionand fogging, best results have been obtained by the application ofcoatings within said range of thickness.

The process of the present invention provides for an unusual andunexpected control of the degree of cross-linking of the polyvinylalcohol which is required to obtain the desired dual properties of (1)resistance to removal of the coating upon wet abrasion and (2)resistance to fogging of the coated surface upon exposure to anatmosphere of high humidity. The desired properties of the coated baseoptical element are obtained by curing the coating to get the desiredwater insolubility of the coating without losing the antifog propertywhich depends upon the partial water solubility of the coating. This isobtained in the process of the invention by utilizing as cross-linkingagents a combination of zirconium (zirconyl) nitrate and formaldehyde.lIt has been found that the use of zirconyl nitrate alone in varyingamounts equal to or over the total amount of an effective combination ofzirconyl nitrate and formaldehyde (on a weight basis) provides anunsatisfactory degree of water insolubility such that the watersolubility is excessive resulting in an unsatisfactory product. On theother hand, the use of a comparable amount of formaldehyde alone hasbeen found to provide too small a degree of cross-linking. When aproportion of formaldehyde equal to an effective amount of zirconylnitrate and formaldehyde in the coating compositions of the inventionwas used, the product, even when it is heated excessively, retainsexcessive water solubility and the coating does not exhibit antifogproperties. In addition, when zirconyl nitrate is used as the onlycross-linking agent in the coating compositions of the invention, anobjectionable brown color is obtained upon heating the coating to theextent required to provide the desired degree of water insolubility.When formaldehyde is used as the only cross-linking agent, it is foundthat the coating develops a cloudiness or haze upon contact withmoisture. The substitution of a prior art dialdehyde cross-linking agentsuch as glyoxal has resulted in inadequate antifog resistance.

The desired wet abrasion and antifog properties of the coated opticalelements of the invention can be determined utilizing test procedures asfollows: For evaluation of wet abrasion, a sample flat safety lens isabraded when totally submerged in distilled water utilizing a one inchdiameter felt abrading pad of the required coarseness. A weight is usedto press the felt pad against the lens to be evaluated so that a totalof 5 pounds per square inch is obtained on the surface of the lens. Thepad is attached to an arm which is made to oscillate utilizing anelectric motor with each oscillation being counted mechanically.Satisfactory wet abrasion is obtained when the coated sample issubstantially unaffected after 300 wet abrasion cycles.

For evaluation of antifogging properties of the coated optical elementsof the invention, a lens under evaluation is placed in a refrigeratorfor 20 minutes in which the temperature is maintained at a temperatureof about 35 to about 40° F. Subsequently, the lenses are placed in anoven maintained at a temperature of about 105° F. The oven designed forthis test is fitted with a slot in the top of the oven to allow the lensto be dropped into the oven without reducing the temperature therein.The front and back of the oven are fitted with windows and a lamp isplaced in the oven so that an observer can look through the windows toobserve fogging. A dish of water is maintained on a shelf in the ovenand compressed air is bubbled through the water to provide a relativehumidity in the oven of about 45 to about 55 percent. Satisfactorypassage of 3 cycles of cooling and heating under the above conditionsare required for satisfactory antifogging properties.

In the process of the invention a polyvinyl alcohol solution is made upto a concentration of between 3 and 5 percent by weight polyvinylalcohol. The curing agents of the invention are present in theproportion of a total of about 0.2 percent by weight to a total of about0.8 percent by weight. The proportion of zirconyl nitrate toformaldehyde can be about 1 to about 3 to about 3 to about 1. Thezirconyl nitrate is available as the salt containing 37 percent water ofhydration while the formaldehyde is available as a 37 percent activesolution. It is to be understood that the above proportions of curingagent are based on a solids basis rather than a wet basis. The coatingsolution has a viscosity as measured by a Brookfield Viscometer at 25° Cof about 100 cps to about 200 cps.

The polyvinyl alcohol solution is applied to the optical element of theinvention preferably by dipping the optical element into the coatingsolution under standard conditions of room temperature and relativehumidity such as 25° C and 40 percent relative humidity. The rate atwhich the optical element is withdrawn from the solution can beimportant in insuring the uniform distribution of the coating on theoptical element of the invention and generally it is desirable toprovide a withdrawal rate for removing the substrate from the coatingsolution which is on the order of about 1/2 to about 3 inches perminute. The coating subsequent to application is dried at roomtemperature for a period of about 15 minutes to about 1 hour andsubsequently cured at a temperature of about 100° C to about 175° C. Thetime for the curing operation and temperature are important in that toomuch time can result in an excessive degree of cross-linking such thatthe coating exhibits little or no antifog properties while too littlecure time can result in a coating having too little resistance to wetabrasion. Thus, the time for cure can be about 2 minutes to about 30minutes depending upon the temperature at which the coating is cured. Asuitable time and temperature for cure is as follows: 121° C for 30minutes.

The following examples illustrate the various aspects of the inventionbut are not intended to limit it. Where not otherwise specifiedthroughout this specification and claims, temperatures are given indegrees cent grade and parts are by weight.

EXAMPLE 1

An antifog coating according to the invention was applied to an opticalelement of allyl diglycol carbonate formed by casting a base lenselement according to procedures known in the prior art. Adequate bondingof the subsequently applied antifog coating of the invention to theallyl diglycol carbonate lens is provided by dipping said lens in anaqueous solution of sodium hydroxide maintained at a temperature of 23°C and having a solids content of 15 percent. Subsequent to immersion ofthe lens in the solution of sodium hydroxide, the lens was air dried at20° C by blowing air under pressure onto the lens and then the lens wasdipped into a solution of polyvinyl alcohol prepared by dissolving 45grams of polyvinyl alcohol sold under the trademark of "Gelvatol 1-90"by Monsanto Chemical Co. in a mixture of 426.5 grams of demineralizedwater and 315 grams of ethanol. The solution is heated to a temperatureof 100° C while under rapid agitation. The agitation is maintained untilthe polyvinyl alcohol is completely dissolved which takes about 20-30minutes. Water lost by evaporation is replaced and the resultingsolution is then filtered while still hot through a 14 micron pressurefilter. Thereafter, approximately 5 drops of a non-ionic surface activeagent sold under the trademark "Triton X-100: by Rohm and Hass Companyis added to the mixture to provide about 0.03 percent of surface activeagent.

To the above prepared polyvinyl alcohol solution there is added 200grams (0.4 percent of the total solution) of zirconium nitrate as a 2percent solids solution of the salt containing 37 percent water ofhydrolysis and 0.5 percent of formaldehyde on an active basis from thecommercially obtainable 37 percent active solution. Thus, a total of 4.0grams of zirconium nitrate is present together with 5.0 grams offormaldehyde as a 37 percent active solution. The solution pH was 3.4and the viscosity was 180 cps when measured on a Brookfield Viscometerat 25° C subsequent to the addition of the zirconium nitrate andformaldehyde solutions.

The polyvinyl alcohol solution prepared above was applied to the allyldiglycol carbonate lens by a dipping operation carried out at atemperature of 25° C and a relative humidity of 40 percent. Thewithdrawal rate during the dipping operation was 11/2 inches per minute.The curing temperature was 121° C and the time was 30 minutes.

A durable, abrasion-resistant antifog coating was thus obtained on anallyl diglycol carbonate substrate.

EXAMPLE 2 (control)

A polycarbonate sold under the trademark "Lexan" was molded into asafety goggle lens and was provided with a coating to provide adequatebonding of the antifogging coating by applying a 10 percent solution inethanol of gamma-aminopropyltriethoxysilane. The polycarbonate lens wascoated by dipping into the solution. The substrate was then air driedand rinsed with water which was subsequently dried using compressed air.Following the process of Example 1, a coating was applied by dipping thelens into a polyvinyl alcohol solution prepared as in Example 1 exceptthat instead of the use of zirconium nitrate and formaldehyde as curingagents for the polyvinyl alcohol solution, 1 percent of glyoxal wassubstituted therefor. The polyvinyl alcohol coating was applied andcured at 121° C for 30 minutes in accordance with the procedure ofExample 1.

Upon evaluation of this coating by testing for antifog resistance inaccordance with the procedure outlined above, the coating fails thefirst cycle by exhibiting fogging. The coating, however, passed thepreviously described wet abrasion test without failure after 300 cycles.

EXAMPLE 3 (control)

The procedure and proportions of Example 2 were used except that 0.7percent zirconyl nitrate was used as the only curing agent in thepolyvinyl alcohol coating solution.

Upon coating and curing, the polycarbonate lens in accordance with theprocedure of Example 1, an objectionable brown color was obtained.

Upon evaluation for antifog properties, the coated lens was found tofail the first cycle of the above-described test, however, the coatedlens passed the above-described wet abrasion test showing no failureafter 300 cycles.

EXAMPLE 4 (control)

The procedure and proportions utilized in Example 2 were used with theexception that 1 percent formaldehyde was used as the only cross-linkingagent in the polyvinyl alcohol coating solution . Upon evaluation of thecoated samples made in this manner, it was found that satisfactoryantifog properties were obtained; the sample passed three cycles of theabove test for antifog resistance. In addition, the coated sample passesthe wet abrasion test without failure after 300 cycles, however, thecoating exhibits an objectionable haze upon contact with water.

EXAMPLE 5

In accordance with the procedure of Example 1, a silicate based glasslens was provided with an abrasion-resistant antifog coating by firstdipping said lens in an aqueous solution of sodium hydroxide maintainedat a temperature of 23° C and having a solids content of 15 percent.Subsequent to immersion of the lens in the solution of sodium hydroxide,the lens was air dried at 20° C by blowing air under pressure onto thelens. The lens was then dipped into a solution of polyvinyl alcoholprepared in accordance with the procedure of Example 1. A durable,abrasion-resistant antifog coating was thus obtained on a silicate glasssubstrate.

EXAMPLE 6

The procedure and proportions of Example 2 were repeated except that thecuring agents of Example 1 were substituted for glyoxal and used in thesame proportions as shown in Example 1.

A durable, abrasion-resistant antifogging coating was obtained on apolycarbonate safety goggle lens.

While this invention has been described with reference to certainspecific embodiments, it will be recognized by those skilled in the artthat many variations are possible without departing from the scope andspirit of the invention.

I claim:
 1. An abrasion-resistant optical element comprising atransparent base element of glass or plastic having a transparent,abrasion-resistant, antifogging coating on at least one surface thereofcomprising a cross-linked polyvinyl alcohol wherein said polyvinylalcohol is cross-linked with a curing agent which is composed of 1 to 3parts by weight of zirconium nitrate and 3 to 1 parts by weight offormaldehyde, said curing agent constituting 0.2 to 0.8 percent byweight, on a solids basis, of the total weight of polyvinyl alcohol andcuring agent.
 2. The optical element of claim 1 wherein said transparentbase element is a polycarbonate.
 3. The optical element of claim 1wherein said transparent base element is allyl diglycol carbonate.